Currently, storage systems each using more than one large-capacity storage device such as more than one HDD (hard disk drive) are widely used. Generally, the storage systems each contain a plurality of storage devices and a control device which controls access to the plurality of storage devices. In some storage systems, the access paths to the plurality of storage devices are redundantly provided by arranging a plurality of control devices in each storage system. The redundancy of the access paths improves the reliability of the access to the storage devices.
Further, redundancy can be realized in the access path between each control device and a storage device by arranging a first access path directly connecting each control device and the storage device and a second access path connecting each control device and the storage device through another control device. For example, in the case where each control device contains an SAS expander which relays access to a storage device, each control device may be arranged to be able to access the storage device by using either of a first access path to the storage device through the SAS expander arranged in the control device and a second access path to the storage device through an SAS expander arranged in another control device. (SAS stands for Serial Attached SCSI, and SCSI stands for Small Computer System Interface.)
In a known example of a storage system in which control devices are redundantly arranged, a router is arranged between a plurality of storage devices and a disk adapter provided in each control device, and the storage system is configured to enable communication by selectively switching the disk adapter and the storage device. Further, in another known example of a storage system in which a plurality of control devices access a storage device through a plurality of external SAS expanders, each control device and each SAS expander have a plurality of physical ports and correspond to one of two port groups, and the control devices, the SAS expanders, and the storage device are connected by appropriately combining the physical ports so as to realize the redundancy. (See, for example, Japanese Laid-open Patent Publications Nos. 2006-155392 and 2006-72636.)
As mentioned before, in the storage systems in which a plurality of control devices are arranged, and a first access path directly connecting each control device and the storage device and a second access path connecting each control device and the storage device through another control device are arranged, it is possible to make the access path to the storage device redundant, and the redundancy of the access path improves the reliability of the access to the storage device. However, when a control device in the second access path fails, the second access path becomes unusable, so that the redundancy of the access path is lost. When the redundancy of the access path is lost, the access reliability is lowered, and access to the storage device is concentrated in the first access path, so that the performance of the access to the storage device is lowered.